PIC16F84A software UART (bit banging) code and Proteus simulation

This post provides the software UART (Bit Banging) code for PIC16F84A microcontroller (e-g to connect PIC controller with PC using serial adapter). As we know, PIC16F84A microcontroller doesn’t have built in UART module, so we can create UART functionality in it’s software. This post provides the details of how to program software UART functionality in PIC16F84A. This code is written in C language using MPLAB with HI-TECH C compiler. You can download this code from the ‘Downloads‘ section at the bottom of this page.

It is assumed that you know how to blink an LED with PIC16F84A microcontroller. If you don’t then please read this page first, before proceeding with this article.

The result of simulating the code in Proteus is shown below.

In the above figure, UART baudrate is currently set to 1200 bps, but you can change it to your desired value. RA0 pin is being used as TX pin and RA1 pin is the RX pin of the software UART. Code is written in such a way that PIC16F84A echos back whatever character is sent to it. Hello World! was typed in the virtual terminal after the start up of the simulation in Proteus and PIC16F84A echoed it back.

Code

The code used to set different properties of UART is shown below. (From Software_UART.h file)

In the above figure, _XTAL_FREQ is defined to be 20000000, which is the external crystal frequency used with this PIC16F84A. If you change the crystal (e-g by attaching an external crystal of different frequency) then you will need to change _XTAL_FREQ value as well. For example, if 12MHz external crystal is used, then _XTAL_FREQ should be defined as 12000000 in the above code.

Similarly, you can define Baudrate for the UART, in the above figure it is defined to be 1200 bps. Since this is a software UART, that is why when you increase Baudrate , then bit error rate for UART also increases drastically. You should increase crystal value to reduce UART bit errors. For example to use a Baudrate of 4800, you should use a crystal of at least 20MHz. DataBitCount is defined to be 8, which means this UART will use one starting bit, 8 data bits and then one stop bit. There is no parity bit and no flow control mechanism.

RA1 and RA0 pins are being used for the UART. You can easily change these pins, for example by defining UART_RX to be RA3 and UART_RX_DIR to be TRISA3, RA3 pin will become UART RX pin. Similarly you can change UART TX pin if required.

The code for main function is shown below.

Downloads

Software UART code for PIC16F84A was compiled in MPLAB v8.85 with HI-TECH C v9.83 compiler and simulation was made in Proteus v7.10. To download code and Proteus simulation click here.